1. Field of the Invention
This invention relates to a volume control apparatus using an electronic attenuator, and more particularly to a volume control apparatus which is capable of adjusting the speed of volume control, i.e., volume increase or decrease rapidly or slowly according to operation of an operating knob.
2. Description of Prior Art
There has been widely known a volume control apparatus using an electronic attenuator as illustrated in FIG. 1. In the figure, 1 is an electronic attenuator having an audio input terminal IN, an audio output terminal OUT and a control terminal C, 2 is a D/A converter for applying a control analog signal to the control terminal C, and 3 is an up/down counter for applying a digital signal to the D/A converter 2 which has a clock input terminal CL, an up/down control terminal U/D and a carry terminal CA where an output appears at a maximum count value and at a minimum count value. 4 is a volume increasing switch comprised of a normally opened contact which is grounded at one end thereof and connected at another end thereof to a power souce B through a resistor R.sub.1. 5 is a volume decreasing switch comprised of a normally opened switch which is grounded at one end thereof and connected at another end thereof to the power source B through a resistor R.sub.2. Junction points between the resistor R.sub.1 and the switch 4 and between the resistor R.sub.2 and the switch 5, respectively, are coupled to input terminals of a logic circuit (NAND circuit) 6, respectively. An output terminal of this logic circuit 6 is coupled to an input terminal of an oscillatory circuit 8 through an inverter 7. An output terminal of this oscillatory circuit 8 is coupled to the clock input terminal CL of the up/down counter 3 through an inverter 9. A junction point between the volume decreasing switch 5 and the logic circuit 6 is coupled to the up/down control terminal U/D of the up/down counter 3. The oscillatory circuit 8 includes a NAND circuit 81 and is adapted to stop oscillation where a signal from the carry terminal CA of the up/down counter 3 is applied to the NAND circuit 81.
The operation of this conventional volume control apparatus will now be described. When the volume increasing switch 4 is closed, a level of an input to the logic circuit 6 becomes low and a level of an output of the logic circuit 6 becomes high. This output is made low by the inverter 7 so that the oscillatory circuit 8 starts oscillation to apply an oscillation output to the clock input terminal CL of the up/down counter 3. The up/down control terminal U/D is at a high level so that the up/down counter 3 counts the oscillation output in the adding direction. The count output is converted into an analog output by the D/A converter 2. A voltage of a signal to the control terminal C of the electronic attenuator 1 is increased and an amount of attenuation by the attenuator 1 is decreased, so that an output from the audio output terminal OUT is increased. On the other hand, when the volume decreasing switch 5 is closed, the oscillatory circuit starts oscillation to apply an oscillation output to the clock input terminal CL and render the up/down control terminal U/D to be of low level. As a result, the up/down counter 3 counts the oscillation output in the subtracting direction. A voltage of a signal to the control terminal C of the electronic attenuator 1 is decreased and the amount of attenuation is increased, so that an output from the audio output terminal OUT is decreased.
As apparent from the foregoing description, in the conventional volume control apparatus, the counting speed of the up/down counter 3 is determined by an oscillation period of the oscillatory circuit 8 and the amount of volume increase or decrease depends upon the closing time of the volume increasing or decreasing switch 4 or 5. Thus, it has not been able for the operator to attain quick or slow volume control as desired.